The sky’s the limit for Tokyo’s newest tower

TOKYO (TR) – After the Great East Japan Earthquake on March 11 last year, the performance of the spectacularly tall Tokyo Sky Tree going up in the capital’s downtown Sumida Ward became a subject of heightened interest to experts, residents and the general public alike.

Tobu Tower Sky Tree, the owner of the project, reported that no structural damage resulted from the magnitude-9.0 quake, which was centered off the coast of Miyagi Prefecture to the north. Credit for this accomplishment can in part be attributed to the temples all the way down in Kyoto.

To the observer, the steel truss exterior gradually tapers from a triangular footprint upwards to a cylindrical spire, today poking through the clouds at its full height of 634 meters. Mostly unseen from the outside, however, is an internal reinforced concrete column.

“The central column is like that used in the five-story pagodas of traditional Japanese architecture,” says Tadano Kamei, a senior architect at architectural firm Nikken Sekkei, which was commissioned for the design work. “The outer frame and central pillar are structurally separate.”

In the event of an earthquake, the upper part of the core column is designed to function as “a balancing weight,” such that when the inner and outer components begin shaking, their relative motions are out of step and oppose one another. A “viscous damping system” similar to shock absorbers, and positioned at various points between the two, dissipates the seismic energy and lessens the swaying. The result is a 50 percent reduction in the structure’s overall movement.

Lower observation deck

Nikken Sekkei claims that not one temple in Japan that utilizes this design concept, known as shimbashira seishin (central column vibration control), has collapsed due to seismic forces.

The arrangement is just one of many special architectural and design considerations utilized for this record-setting radio and television transmission tower, scheduled to open to visitors on May 22.

Resembling a mechanical pencil pointing to the heavens, the Sky Tree, built by construction company Obayashi, is the second-tallest structure in the world after the 828-meter Burj Khalifa in Dubai. Last November, Guinness World Records recognized it as the world’s tallest tower.

For tourists looking to get a bird’s-eye view of the metropolis, two observation decks are positioned 350 and 450 meters from the ground. High-speed elevators, rolling outside the concrete core that houses an emergency staircase of 2,523 steps, zoom up to the lower platform in 50 seconds, and reach the next deck a half-minute later.

Truss network

Kamei says that the intention from the start, in February 2005, was to create a structure that “transcends time and space” through a design that allows the tower to function not only as a transmitter and an observatory “but also an everlasting symbol of Tokyo.”

Such a lofty goal got started with the lofting of balloons. Nikken Sekkei’s team floated 50 balloons from the site to understand wind patterns.

The company had previously designed many tall towers, including, under the direction of Waseda University’s Tachu Naito, the 333-meter-tall Tokyo Tower, completed in 1958.

Tokyo Tower, the current transmitter of TV signals for the city, which will still be in use until 2013, was built to withstand a wind speed of 90 meters per second at its top, but Tokyo Sky Tree is nearly double that height — a crucial factor considering that wind speed generally rises with increasing altitude. “Understanding patterns over the site was essential in preparing a wind-resistant design for this tower,” says Kamei.

To mitigate winds at the top, a “tuned-mass damping” system has been installed. In practice, this translates into two massive ballast weights, one 25 tons and the other 40 tons, that were supplied by Mitsubishi Heavy Industries and suspended near the top with large springs and dampers. Similar to the concept employed with the concrete core and outer truss frame below, these two counterweights work to offset lateral movement.

Back on terra firma, the tower’s foundation is “anchored to the ground like a giant tree,” as the Nikken Sekkei Web site explains. Sets of cylindrical steel and thin-walled concrete piles extend up to 50 meters beneath the surface in an arrangement similar to a combination of the spikes of a baseball and golf shoe.

All told, the criteria for ensuring safety from a natural disaster is impressive: the tower has been built to withstand a magnitude 6.9 earthquake sourced from an unknown fault directly under the site; Tokaibane’s Watanabe says that his company was required to take into account Typhoon Muroto, which struck the Kansai area on September 21, 1934 with powerful winds that killed over 2,700 people.

Upper observation deck

Videos on YouTube shot from 2:46 p.m. on March 11 of last year show the then 625-meter-tall tower swaying, with one of its tall cranes used in the construction process swinging violently. Shigeaki Tabuchi, site director for Obayashi, estimates for the Yomiuri Shimbun (Mar. 1) that the top of the tower was displaced by between four and six meters. Yet the only substantial problem for the project was a two-month delay to the construction schedule caused by delayed material deliveries brought about by supply chain problems.

Of the Sky Tree’s appearance, considerable attention was paid to “lightness of volume” to reduce the amount of steel required and any feeling of oppressiveness to the eyes of local residents. High-strength steel pipes welded vertically, horizontally, and diagonally compose the visible external truss and provide something of an elongated Death Star look.

Kamei says that a structure in which a cross section in the shape triangle transitions to a circle exists nowhere else in the world. The constraints of the property necessitated such an approach.

“Sometimes, unique designs are not rational from a structural viewpoint,” says Kamei of an architect and his limits. “A tower exceeding 600 meters in height has to be structurally rational if we take into consideration the budget,” which as of the latest data available, for 2010, was 59.6 billion yen.

Tokyo Sky Tree

Tokyo Tower presently is the symbolic king of the skyline, with a design that has provided inspiration for books and films — and served as target practice for Godzilla.

Will future generations find the Tokyo Sky Tree to be similarly appealing?

Kamei envisions an appreciation not unlike that for the pyramids of Giza, the towers of San Gimignano in Italy, or the Gateway Arch in St. Louis, Missouri. “I think people tend to preserve monuments which have original shapes and characteristics,” the architect says.

For its part, the Sky Tree has already proved to be a facilitator of dreams. In February, Obayashi, motivated by its work on the project, unveiled plans to build a “space elevator” by the year 2050. Such a contraption, first conceived in the nineteenth century, sends a compartment traveling along the outside of heavy-duty carbon cable, much like a beanstalk, from earth into space.

Yoshio Aoki, a professor within the Department of Precision Machinery Engineering at Nihon University and the director of the Japan Space Elevator Association, sees the Sky Tree as a test case for propelling a manned lift to the stars.

“For elevators in high towers, like the Sky Tree, the hoist ways are not wrapped by walls,” says the professor of what would be the ultimate pie, or castle, in the sky. “This is the same as a space elevator.”

Aoki says that if the elevators in the Sky Tree — which are in glass-enclosed shafts on the outside of the inner core — can endure wind, rain and other elements, and can function continuously, then the concept can be applied to a space elevator.

“That will be the proof we need,” he says.

So, for the “Son of Sky Tree,” it seems, the sky really may not be the limit.

Note: This report originally appeared in the May 20 issue of the Japan Times.